Photodynamic therapy (PDT) uses medicine that is activated with light to kill cells. It is used to treat an eye disease called macular degeneration, some cancers, and skin problems such as acne.

For PDT, a medicine, called the photosensitizer, is put near the cells that need to be destroyed. The photosensitizer may be put on the skin, taken by mouth, or given in a vein. Then the photosensitizer is "turned on" (activated) with light. The light used is often from a laser or from light-emitting diodes (LEDs). The specific wavelength of light activates the medicine to make a kind of oxygen that kills nearby cells.

Photosensitizers will make the skin and eyes sensitive to light for about 6 weeks after treatment. So people who have PDT need to avoid direct sunlight and bright indoor lights.